This invention relates to a house arrest monitoring system having improved tamper detection.
The increasing use of house arrest sentences as a substitute for incarceration has generated a need for reliable monitoring systems. To accomplish the primary purpose of checking to see if the monitored subject is abiding by the terms laid down by the judicial system, the personal monitoring system must not only be reliable, but also not capable of being easily defeated. The typical system is comprised of a securing strap which affixes an electronics module to a limb of the subject. The strap includes a conductor which completes an electrical circuit. Attempts to remove the attachment from the subject generally require that the strap be severed or decoupled from the module.
The ingenuity of monitored subjects has resulted in the defeating of monitors which rely solely on the detection of a change in the impedance of conductors in the strap. For example, the immersion of a portion of the strap in an ionic liquid such as salt water followed by decoupling has been found to defeat many of the monitoring systems. The subject having successfully tampered with the monitor is then free to move about and replace the monitor at his convenience. To avoid the defeating of the system by an immersion technique, there have been proposed a number of systems utilizing electrical reactance measurements between a pair of conductors in the strap to determine a tamper condition. Also, attempts have been made to utilize the characteristics of the body of the subject such as skin temperature to establish a normal operating condition. Any change in the monitored quantity detected over a number of tests indicates a tamper. The electrical measurement of body characteristics call for complex circuitry and leads to unreliable performance.
Accordingly, the present invention is concerned with the provision of an electric module and a securing strap for a monitoring condition which utilizes a plurality of conductors in the strap and establishes a voltage pattern for the conductors in the strap. Any variation in the pattern from that established is readily determined in the module to provide an indication of the tamper condition. The present system establishes a pattern of open and closed circuit conditions for the conductors in the strap. The pattern is preferably varied over time to defeat attempts to avoid detection by the bridging of conductors in the strap. In addition, the monitoring system is responsive to an optical reset signal after tampering only after the reset circuit has been initialized. Initialization is provided by a contacting device which is operated by the monitoring agent or officer.